1. Field of the Invention
The present invention relates to an object sensor and an object sensing method for sensing an object such as a water drop that exists on a sensing surface of a product. Especially, it is related to a rain sensor and a rain sensing method applied to vehicles, and a windshield wiper controller and a windshield wiper control method using the object sensor.
2. Description of the Related Art
Conventionally, many types of rain sensors for vehicles are developed. For example, a rain sensor employing an optical method as follows is known. A light emitted from a photo emission element (light emission element) is reflected on a sensing surface and the reflected light is received by a photo detector, and then a rain drop is sensed. That is, the reflection condition varies if an object such as a water drop exists on the sensing surface, and the amount of the detected light by the photo detector will decrease. A conventional rain sensor senses a rain drop by detecting this variation.
The above mentioned varying detection, normally, a method for comparison with the reference value (threshold method) had been used (i.e. JP 10-186059 A).
In practical use of such a conventional rain sensor, the rain sensor will be used in various conditions, so that a means for preventing malfunction is necessary. In order to achieve this object, the conventional rain sensor uses plural reference values which are set according to the operation modes (JP 10-186059 A), or replaces and switches reference values one by one (JP 2-68248 A).
As mentioned above, with the conventional rain sensor, the logic for rain drop detection is complicated, and consequently, a high-speed processing for of rain drop detection becomes difficult. In addition, in either of the above mentioned conventional methods, the rain drop is detected by the comparison with the reference value based on judging the status on the sensing surface. Therefore, the prevention of the malfunction is difficult because of the influence of the light from the outside or the influence of the status of the sensing surface, e.g. where dirt is present.
Moreover, the photo emission element and the photo detector have a disadvantage that the element characteristic will vary when the temperature varies. Especially, the photo emission element such as LED has a characteristic that the output will decrease when the temperature rises. By this characteristic, such element has a problem that an appropriate detection cannot be processed unless data correction by monitoring and feeding back the output is processed.
For example, in JP 57-118952 A, a windshield wiper controller is disclosed. This apparatus employs a method in which a light emitted from a photo emission element and reflected on the surface of a windshield glass is received by a photo detector, then the amount of the rain drop is judged according to the received light signal level, and the windshield wiper is driven.
In more detail, the received light signal is detected, then the detected signal is provided to the differential circuit and pulse signals corresponding to the envelope of the received light signal are picked out, and these pulse signals are counted.
In this JP 57-118952 A, the detailed method of how to pick out the pulse signals is not clearly disclosed. However, judging from FIG. 3(c) of the publication, the system will pick out a pulse signal when detecting the intersection of the envelope and the threshold.
In short, in the technology disclosed in the JP 57-118952 A, a rain drop impacting the surface of the windshield may be detected and counted and the detection for impact of a rain drop is processed by comparison the envelope with the threshold.
Therefore, with the foregoing in mind, the object of the present invention is not to judge the status of the sensing surface by comparison of the signal from the sensing surface with the reference value. The present invention provides an object sensor and a object sensing method that can detect the dynamic impact of a rain drop and does not require the complex judgment logic and also provides a windshield wiper controller using the object sensor.
Moreover, the object of the present invention is to provide an object sensor and a object sensing method that can detect a rain drop appropriately even though the signal level fluctuates and provide a windshield wiper controller using the object sensor.
In order to solve the above-mentioned problems, the first object sensor sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: a means for generating a time lag signal from the output signal of the photo detector; a means for calculating a differential signal between the output signal of the photo detector and the time lag signal; and a means for judging the status of the sensing surface by detecting the generation of the differential signal.
According to the above mentioned configuration, the status of the sensing surface can be detected by evaluating the variation over time of the signal level, not evaluating the signal level itself of the photo detector.
In order to solve the above-mentioned problems, the second object sensor sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: a means for canceling noise in a signal outputted from a photo detector; a means for generating a time lag signal from the noise-cancelled signal; a means for calculating a differential signal between the noise-cancelled signal and the time lag signal; and a means for judging the status of the sensing surface by detecting the generation of the differential signal.
According to the above configuration, the status of the sensing surface can be detected by evaluating the differential signal between the variation over time of the signal from the noise canceling processing and the variation over time of the first order time lag signal generated from the noise-cancelled signal, not by evaluating the signal level itself of the photo detector, so that the small fluctuation such as noise can be eliminated and only the significant varying of the signal level can be detected based on the status on the sensing surface.
In order to solve the above-mentioned problems, the third object sensor sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: a means for generating a first order time lag signal from the output signal of the photo detector; a means for generating a second order time lag signal from the first order time lag signal; a means for calculating a differential signal between the first order time lag signal and the second order time lag signal; and a means for judging the status of the sensing surface by detecting the generation of the differential signal.
According to the above configuration, the status of the sensing surface can be detected by evaluating the differential signal between the first order time lag signal and the second order time lag signal, not by evaluating the signal level itself of the photo detector. Because of evaluating the differential signal between the first order time lag signal and the second order time lag signal, the small fluctuation such as noise can be eliminated and only the significant varying of the signal level can be detected based on the status on the sensing surface.
It is preferable that in the above mentioned first, second or third object sensor, said means for canceling the noise and/or said means for generating a time lag signal are/is an analog circuit for generating the time lag signal.
It is preferable that the object sensor further comprises an A/D conversion means for converting an analog signal to a digital signal by sampling the analog signal of the photo detector by a predetermined period; wherein said means for canceling the noise and/or said means for generating a time lag signal are/is a means for leveling the predetermined number of the sampled signal inputted sequentially from the A/D conversion means.
According to the above configuration, a simple analog circuit element or a simple digital processor can be used as a means for canceling a noise signal or a means for generating a time lag signal, so that the circuit configuration becomes simple.
It is preferable that in the above mentioned first, second or third object sensor, the object to be sensed is the impact of a water drop on the sensing surface.
The object to be sensed is a water drop, so that the object sensor of this invention can be utilized as a water drop sensor for sensing the impact of a water drop on the sensing surface.
It is preferable that in the above mentioned first, second or third object sensor, wherein said object to be sensed is an impact of a water drop on the sensing surface, when the differential signal is calculated as a value by subtracting the signal of the photo detector or the signal of the noise-cancelled signal from the time lag signal, the impact of a water drop on the sensing surface is sensed when a positive differential signal is generated.
According to the above configuration, the differential signal is evaluated as a value by subtracting the signal of the photo detector or the noise-cancelled signal, and the impact of the water drop can be detected surely if the significant, positive differential signal which is generated between both signals in the part where the decrease of the signal level by the impact of the water drop is seen.
It is preferable that in the above mentioned third object sensor, wherein said object to be sensed is a impact of a water drop on the sensing surface, when the differential signal is calculated as a value by subtracting the first order time lag signal from the second order time lag signal, the impact of a water drop on the sensing surface is sensed when a positive differential signal is generated.
According to the above configuration, the differential signal is evaluated as a value by subtracting the first order time lag signal from the second order time lag signal, and the impact of the water drop can be detected surely if the significant, positive differential signal, which is generated between both signals in the part where the decrease of the signal level by the impact of the water drop, is seen.
It is preferable that the size of the water drop impacting the sensing surface is judged by the value of the differential signal. This becomes possible because the larger the size of a water drop, the bigger the level of the decrease of the signal level of the photo detector caused by impact of the water drop on the sensing surface.
It is preferable that the spike noise is cancelled precedingly from the signal of the photo detector.
In the above mentioned description, an object to be sensed is assumed to be a water drop, in addition, a water drop is assumed as a rain drop, the object sensor can sense whether it is raining or not.
A windshield wiper control system of the present invention controls a windshield wiper operation by the signal outputted from the object sensor of the above mentioned present invention.
According to the above configuration, the windshield wiper control system can sense a rain drop with high accuracy and perform windshield wiper operation control automatically.
In order to solve the above-mentioned problems, the first object sensing method of the present invention for sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: generating a time lag signal from the output signal of the photo detector; calculating a differential signal between the output signal of the photo detector and the time lag signal; and judging the status of the sensing surface by detecting the generation of the differential signal.
In order to solve the above-mentioned problems, the second object sensing method of the present invention for sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: canceling a noise in a signal outputted from a photo detector; generating a time lag signal from the noise-cancelled signal; a circuit element for calculating a differential signal between the noise-cancelled signal and the time lag signal; and judging the status of the sensing surface by detecting the generation of the differential signal.
In order to solve the above-mentioned problems, the third object sensing method of the present invention for sensing the status of a sensing surface by detecting a light with a photo detector, which light has been emitted by a photo emission element and reflected on the sensing surface, comprises: generating a first order time lag signal from the output signal of the photo detector; generating a second order time lag signal from the first order time lag signal; calculating a differential signal between the first order time lag signal and the second order time lag signal; and judging the status of the sensing surface by detecting the generation of the differential signal.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.